The invention relates to compositions of polyolefin powders, in particular polyethylene powders, having improved adhesion to substrates and to objects having outer coatings made from such compositions.
Polyolefin plastics are more or less inert because of their structure. Polyolefins, such as polyethylene and polypropylene, are now produced on a very large scale. Polyethylene is generally more inert than polypropylene. Therefore polyethylene is difficult or impossible to bond to substrates. It hardly adheres or does not adhere at all to substrates, such as metal surfaces, foamed plastics or the like. To make polyethylene adhere to such a substrate adhesives must be employed, or the surface of the polyethylene and/or the substrate must be etched. However, using adhesives has achieved only a limited degree of adhesion because of the generally poor bonding properties of polyethylene. Moreover, adhesives require extra treatment, which increases the cost and price. Etching of one or both of the surfaces to be bonded together is a laborious process, which also lengthens production time and increases the cost price. Similar difficulties are encountered where polypropylene is employed.
It is already known that shaped objects can be made from plastics by heating powders in a mold. The powder melts on the hot wall and forms a layer. The surfaces of the objects to be made may be thus formed by a single layer or by several layers. Foam structures in between layers or against a layer may also be employed to improve insulating properties or to limit the combined weight of the shaped objects. The surface layer and the foamed or unfoamed layer in contact therewith should, of course, adhere to each other. However, particularly with polyethylene the present levels of adhesion leave much to be desired. The same holds true for polypropylene as well.
This problem has been encountered particularly in the manufacture of surfboards, which are mostly made by rotational molding to form an outer wall of polyethylene. In particular, polyethylene having high or medium density, i.e. a density of at least 0.930 g/ml has been employed therein. It is also possible to use lower density polyethylene. A polyurethene foam fills the interior space defined by the surface layers or wall formed on the mold surface.
During use, however, the outer polyethylene outer wall may be damaged. Water can then penetrate into the foam itself. If there is good adhesion between the outer wall and foam then damage is less likely to occur. In this latter case water can penetrate into the foam only under the damaged spot, whereas if adhesion is poor, water can be distributed between the wall and foam and can thus be absorbed by all of the foam. Good adhesion between the outer polyethylene skin layer and the polyurethane cover is highly desirable for surfboard in particular, but is also important for other objects built up of similar components. Consequently, there remains a definite need for good adhesion.
U.S. Pat. No. 3,639,189 proposes to improve the adhesion of polyethylene to metal substrates by blending the polyethylene with oxidized polyethylene. The oxidized polyethylene is obtained by heating polyethylene in an oxygen atmosphere at temperatures that may range from about 90.degree. C. up to the crystalline melting point of the polyethylene until the desired degree of oxidation has been attained. Next, the unoxidized polyethylene is melt-blended with the oxidized polyethylene, and the mixture is then granulated. The thus obtained granulate is used to form the layers on a metal substrate by method mentioned above. However, polyethylene oxidation is a laborious process and increases the cost and eventual price of the composition employed as a starting material to produce coated polyethylene shaped objects.
A composition of this kind is also undesired in the manufacture of surfboards, for example, because oxidized polyethylene lowers the resistance of the polyethylene composition to atmospheric influences. Notably the resistance thereof to thermal and oxidative attack is lowered. In particular, this is highly undesirable for an outer wall, such as the polyethylene coating or a surfboard, which is exposed to outdoor atmospheric influences. This drawback might be overcome by introducing more stabilizer additives in the polyethylene composition employed. However, adding stabilizer also increases the cost and price.
Moreover, compositions containing oxidized polyethylene when used to manufacture objects in a mold by, for instance, rotational molding, have another disadvantage. These compositions strongly adhere to the mold surface itself which makes it difficult to remove the finished article. Even if a non-oxidized polyethylene is used, it is still necessary to apply release agents in order to facilitate the release of the polyethylene object from the mold. When oxidized polyethylene is used difficulties are still encountered in removing the object from the mold even if release agents are employed.
It has already been attempted to improve properties as well as adhesion by applying a cross-linked polyolefin, by preference a cross-linked polyethylene, or by cross-linking the polyethylene after it has been formed into a surface layer. Improved adhesion proved to be possible, but a disadvantage remained. Damage to the surface layer remained difficult or impossible to repair.
It has now been found according to the present invention that improved adhesion of polyolefins, in particular polyethylene, to another plastic substrate, such as polyurethane foam, can be achieved without polyethylene or another polyolefin strongly adhering to the mold wall during the molding process by using a polyolefin composition having about 20% to about 99.5% by weight of a stabilized polyolefin powder and about 0.5% to about 80% by weight of an unstabilized or hardly stabilized polyolefin powder having a crystalline melting point that is at least 1.degree. C. higher than that of the stabilized polyolefin. By preference the crystalline melting point of the unstabilized or hardly stabilized component is at least 4.degree. C. higher than that of the stabilized component. At the same time adherence of the polyolefin to the mold surface during the molding operation is substantially reduced.